In typical reproduction devices, such as copiers or printers, for example, information is reproduced on individual cut sheets of receiver material such as plain bond or transparencies. Receiver sheets of the various types are stored in stacks and respectively fed seriatim from such stacks when copies are to be reproduced thereon. The sheet feeder for the reproduction devices should be able to handle a wide range of sheet types and sizes reliably and without damage. Desirably, the sheets are accurately fed individually from the sheet stack without misfeeds or multifeeds.
Reproduction device sheet feeders are typically of two types, vacuum feeders or friction feeders. An exemplary vacuum sheet feeder is shown in U.S. Pat. No. 5,344,133, issued Sep. 6, 1994, in the name of Jantsch et al. In such an apparatus, a stack of sheets is stored in a supply hopper. A sheet feed head assembly, including a plenum, a vacuum source in flow communication with the plenum, and a mechanism, such as a feed belt associated with the plenum, transports a sheet acquired by vacuum in a sheet feeding direction away from the sheet supply stack.
Typically, in most vacuum sheet feeders, the sheet supply stack is supported to maintain the topmost sheet at the feed head assembly. A first positive air supply then directs a flow of air at the sheet supply stack to levitate the top several sheets in the supply stack to an elevation enabling the topmost sheet to be acquired by vacuum from the sheet feed head assembly plenum. Additionally, a second positive air supply typically directs a flow of air at an acquired sheet to assure separation of any additional sheets adhering to such topmost sheet.
It is clear that the sheet stack should be maintained in a particular positional relation with the sheet feed head assembly to assure desired feed from the stack. An exemplary control of a sheet stack is shown in U.S. Pat. No. 5,823,527 issued Oct. 20, 1998, in the name of Burlew et al. In such an apparatus, a sheet feeder is disclosed having a platform for supporting a stack of sheets, a feed head assembly for feeding sheets seriatim from the top of a sheet supply stack on the platform, a mechanism for moving the platform relative to the feed head assembly, and device for controlling operation of the platform moving mechanism. The control device can determine a selected parameter in response to examination of sheet stack parameters, and consequently produce a signal corresponding thereto. The speed of the platform moving mechanism is then set based on the parameter signal.
Modern reproduction devices have more than one sheet feeder to store different types of sheets. When running large print jobs without any stop page there is a need to switch over from one feeder to another. Normally the first stack is not run empty before switching over to the next stack. It is preferred to leave the minimum number of sheets necessary to insure that the feed source will not run out prior to switching. This maximizes the effective capacity of the supplies and minimizes the number of sheets that are likely to be exposed to undesirable environments for an extended period of time as a result of being left behind. Normally, feeding is switched to another feed source when a paper low condition is signaled. This is typically determined by sensing that the platform has reached a certain position, either through action of a switch, or feedback from a platform travel monitor, such as an encoder, potentiometer or step count from a step motor. The actuation point for this paper low condition is selected to insure that a sufficient number of receiver sheets is present to allow switching under all conditions. Due to the system architecture, the system tolerances and differences in the receiver sheet thickness, this actuation point is selected conservatively. This results in an excessive number of sheets remaining under most conditions.
The stack advancing is often performed with stepper motors. The height position of the stack is proportional to the number of steps a stepper motor is triggered. The paper supply controller needs data relating to the displacement of the stack supporting platform relative to a down switch for several reasons. The displacement data is used to determine the paper low status as well as enabling the paper out check and other functions. The paper low displacement is one parameter that determines how many sheets are left behind in a supply hopper after a continuous mode swap, wherein paper supplies are switched and filled alternately in order to provide continuous stream of sheets to the marking engine. As mentioned before, the displacement can be measured in terms of stepper motor steps applied. The mechanical tolerances in the stack advancing mechanism are such that no nominal value for each of these displacements would give an acceptable performance for all supplies of the reproduction apparatus. Although it is possible to manually calibrate the total possible displacement of an elevator, it is inconvenient to manually calibrate for paper thickness.
The embodiments described herein allow for more effectively controlling the level of a sheet stack and the switching over to the next stack.